Currently, a Study Item on LTE working on unlicensed band (LTE-U) has been initiated in 3GPP Rel-13 to standardize this feature. The first stage of LTE-U would consist of using the unlicensed band for supplementary downlink (SDL) and the uplink is sent using the licensed band. Since LTE-U operates in the unlicensed band, it would have to co-exist with other networks, e.g. other LTE-U and Wi-Fi (e.g. 802.11ac).
The hidden node is a well-known problem in FIG. 1 shows two Wi-Fi Access Points (AP), AP#1 and AP#2. Terminal 1 is connected to AP#1 whilst Terminal 2 is connected to AP#2. AP#1 cannot hear the transmission from AP#2 but Terminal 2 can hear both AP#1 and AP#2 transmission. Whilst AP#2 is transmitting data to Terminal 2, AP#1 failing to sense AP#2 transmission may transmit data to Terminal 1, which causes interference to Terminal 2.
Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) is introduced in Wi-Fi to mitigate the hidden node problem. This is shown in FIG. 2, where AP#2 transmits a “Request to Send” (RTS) to Terminal 2 prior to transmitting any actual data. Any nodes detecting the RTS would refrain from transmission. In response to the RTS, Terminal 2 would sense the channel and if it is free, it would response with a “Clear to Send” (CTS) back to AP#2. Similarly any node that can detect this CTS will refrain from transmitting. In FIG. 2, AP#1 would detect the CTS and would not transmit data to Terminal 1. Upon receiving the CTS, AP#2 would start its data transmission to Terminal 2. In this way the Terminal 2 would avoid being interfered by AP#1.
In existing 802.11ac Standards, the RTS contains a single destination address. However, in LTE, the eNB can schedule multiple UEs within a single sub-frame. This feature should be maintained in LIE-U when employing CSMA/CA mechanism in LTE-U. Specifically, when RTS is used to prevent Hidden Node interference as described above.
The CSMA/CA mechanism can be adopted for LTE-U to mitigate collision between two or more LTE-U network and between LTE-U and WiFi networks. The technical problem to solve is to extend the RTS-CTS to multiple LIES to maintain simultaneous downlink data transmission to multiple UEs in LTE-U. This shall improve upon the single recipient of the existing RTS frame design of 802.11ac.